feat(observability): Phase 6 OTLP tracing + Prometheus metrics endpoint
OTLP tracing:
- libs/observability/otlp.rs: SdkTracerProvider via HTTP/proto OTLP exporter
- libs/observability/tracing_middleware.rs: Actix-web span with trace_id propagation
- libs/observability/tracing_fmt.rs: JSON fmt + registry.try_init for layered init
- libs/rpc: gRPC method spans via info_span
- libs/agent, libs/room, libs/service, libs/api: structured tracing throughout
Prometheus metrics:
- libs/observability/prometheus_exporter.rs: /metrics HTTP handler + metrics crate
- libs/observability/metrics_middleware.rs: HttpMetrics middleware + AtomicU64
- libs/observability/redis_metrics.rs: Redis counter poller via RedisMetrics
- libs/room/metrics.rs: RoomMetrics (connections, messages, presence counters)
Config env vars: APP_OTEL_ENABLED, APP_OTEL_ENDPOINT, APP_OTEL_SERVICE_NAME
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